There exists a number of biological wastewater treatment processes for the removal of COD, phosphorous and nitrogen from wastewater utilizing microorganisms contained in an activated biomass, or sludge. These treatment processes typically incorporate multiple treatment zones, namely: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area.
Preliminary treatment is primarily concerned with the removal of solid inorganics from untreated wastewater. Typically, this preliminary treatment encompasses a two-stage treatment process in which the debris is removed by screens and/or settling. Organic matter is carried out in the fluid stream for subsequent treatment.
Primary treatment entails a physical process wherein a portion of the organics, including suspended solids such as faeces, food particles, etc. is removed by flotation or sedimentation.
Secondary treatment typically encompasses a biological treatment process where microorganisms are utilized to remove remaining organics, nitrogen and phosphorous from the wastewater fluid stream. Microorganism growth and metabolic activity are exploited and controlled through the use of controlled growth conditions.
In large scale or industrial applications, this process typically consists of a basin in which the wastewater is mixed with a suspension of biomass/sludge. Subsequent growth and metabolism of the microorganisms, and the resultant treatment of the wastewater, is carried out under aerobic and/or anaerobic/anoxic conditions.
In most large scale municipal or industrial treatment systems, the various components of the treatment process are performed in discrete basins or reactors. As such, there is a continuous flow of the wastewater from one process step to the next. Biomass containing the active microorganisms may be recycled from one process step to another. The conditioning of such biomass to enhance growth of particularized subgroups of microorganisms possessing a proclivity for performing a specific type of metabolic process, e.g. phosphate removal, nitrogen removal has been the subject matter of numerous patents, including: U.S. Pat. No. 4,056,465; U.S. Pat. No. 4,487,697; U.S. Pat. No. 4,568,462; U.S. Pat. No. 5,344,562.
The optimization of other components or aspects of the biological wastewater treatment process has also engendered a variety of patents, including: U.S. Pat. No. 2,788,127; U.S. Pat. No. 2,875,151; U.S. Pat. No. 3,440,669; U.S. Pat. No. 3,543,294; U.S. Pat. No. 4,522,722; U.S. Pat. No. 4,824,572; U.S. Pat. No. 5,290,435; U.S. Pat. No. 5,354,471; U.S. Pat. No. 5,395,527; U.S. Pat. No. 5,480,548; Canadian Patent # 1,064,169; Canadian Patent # 1,096,976; Canadian Patent # 1,198,837; Canadian Patent # 1,304,839; Canadian Patent # 1,307,059; Canadian Patent # 2,041,329.
The Sequencing Batch Reactor (SBR) process is a modification of the conventional activated sludge process. The SBR process employs a number of discrete steps comprising the sequential fill, reaction, settlement and decantation of wastewater with biomass in an enclosed reactor. In the initial step of this process, wastewater is transferred into a reactor containing biomass, and combined to form a mixed liquor. In the reaction step of the treatment process the microorganisms of the biomass utilize and metabolize and/or take up the nitrogen, phosphorous and organic sources in the wastewater. These latter reactions may be performed under anaerobic conditions, anoxic conditions, aerobic conditions, or a combination thereof.
Following the reaction cycle, the biomass in the mixed liquor is allowed to settle out. The treated and clarified wastewater (i.e. effluent) is subsequently decanted and discharged. The reactor vessel is then refilled and the treatment process cycle reinitiated.
SBR's have been successfully used to treat wastewater generated by small communities.
A common factor in all of the aforementioned systems for wastewater treatment is the prerequisite of a fairly consistent inflow of wastewater for the maintenance of optimal treatment capability. However, the treatment problem presented by wastewater generated at cottages, or other sporadic or seasonally lived-in communities, is markedly different from that of a permanent residence due to the highly intermittent or sporadic generation and flow of wastewater.
In addition to the usual diurnal fluctuations, extreme weekly and seasonal fluctuations are expected, wherein flows will vary from nil to several times that normally expected for a single family residence. Whereas high flow events can be dealt with hydraulically, using equalization capacity, the situation is quite different for periods of no flow. During these periods the microorganisms which normally facilitate the conversion of wastes will be starved.
Recent studies of bacteria under starvation conditions have established that many microbes are capable of withstanding long periods of starvation (Kjelleberg S., Albertson N., Flardh K., Holmquist L. Jouper-Jaan A., Marouga R. & Osthng J. 1993. "How do non-differentiating bacteria adapt to starvation", Antoine van Leeuwenhoock 63: 333-341). In fact, this is considered to be the normal situation in nature (Morita R. Y. 1982. "Starvation-survival of heterotrophs in the marine environment", Adv. Microb. Ecol. 6: 171-198). However, these studies have involved either marine isolates or pure cultures, and so do not reflect either the environment or the ecosystem diversity present in actual sewage treatment systems.
Other studies involving starved bacteria have focused on inactivation of fecal bacteria in marine environments. These findings are not predictive of the starvation-survival in a wastewater treatment system.
Starvation in wastewater treatment systems has also been specifically examined. Chudoba et al. (Chudoba P., Chevalier J. J., Chang J. & Capdeville B. 1991. "Effect of anaerobic stabilization of activated sludge on its production under batch conditions at various So/Xo ratios", Wat. Sci. Technol. 23: 917-926) subjected return activated sludge to 8 hours of anoxia and starvation. Ford and Ekenfelder ((1967) "Effects of process variables on sludge floc formation and settling characteristics"; Journal WPCF 39: 1850-1859) studied the effects of up to 72 hours anoxic starvation on chemical oxygen demand (COD) uptake and oxygen uptake rate for the purpose of observing the effects on mixed liquor floc formation and settling. These studies demonstrated that the mixed liquor from aerobic treatment systems was capable of surviving relatively short periods of starvation and anoxia. These studies are directed at the ability of biomass to survive short term starvation. These studies do not provide any information about the design of a recovery response of an aerobic treatment system which has been starved for several weeks or months or even if it is possible.
The present practice of using septic systems to treat domestic sewage from cottages introduces soluble reactive phosphorous (SRP or PO.sup.3-.sub.4 --P) into the soil. Although the time required for the phosphorous to migrate from the tile bed into the nearby surface waters will vary considerably due to the varying geochemical factors involved, all of the septic SRP will eventually reach surface waters (Dillion P. J. & Molot L. A. 1996. "Long term phosphorus budgets and an examination of a steady-state mass balance model for central Ontario lakes", Wat. Res. 30(10): 2273-2280). As such, a biological wastewater treatment processes offer the potential capability of removing SRP in addition to the concurrent removal of organics and nitrogen from cottage wastewater.
Therefore, there is a clear need for a biological wastewater treatment process in which the biomass/sludge is capable of withstanding relatively long periods of starvation, possessing the ability to subsequently recover biological wastewater treatment activity when restarted.